1. Field of the Invention
This invention relates to apparatus for producing duplex copies. More particularly, this invention relates to apparatus for transferring unfixed transferable images to both sides of a copy sheet before fixing of either image to the copy sheet.
2. Description of the Prior Art
Several techniques are known in the prior art for forming duplex images on a final copy medium such as a web or sheet. One such technique requires the use of two photoconductors upon which first and second transferable images ae formed respectively. The copy medium is generally passed between the photoconductors and the first and second images are transferred to opposite sides of the copy medium. Another technique similar to the above but involving the use of only one photoconductor, utilizes an intermediate image storage member such as a drum, to receive the first image formed on the photoconductor before transfer to a final copy medium. These techniques suffer the disadvantages of increased cost, machine complexity and size and decreased reliability necessitated by the use either of two photoconductive and optical systems or of additional components before transfer to a final copy medium.
A further duplexing technique utilized in certain commercial electrophotographic machines utilizes a "two-pass" process wherein first fixed images developed sequentially on the first sides of a plurality of copy sheets by an electrophotographic process are collected in an intermediate tray and then sequentially transported back through the electrophotographic process to develop second fixed images on the second sides of the copy sheets, thus producing duplex copies. This process is disadvantageous for several reasons. First, duplex copy is not available for inspection until all of the first sides of the copy sheets and one set of the second sides thereof have been developed. In addition, the relatively long paper paths required in passing a copy sheet through the electrophotographic process twice greatly increases the possiblity of paper jams and other potential copy handling complications. Moreover, environmental conditions of image formation and the physical parameters of the copy sheet may change resulting in images of varying quality on opposite sides of a single sheet and misregistration between images on opposite sides of a copy sheet may also result.
Another duplex copying technique involves fixing images to both sides of a copy sheet during a single pass through the disclosed electrophotographic processes. U.S. Pat. Nos. 3,506,347; 3,672,765; 3,869,202; and 3,947,270 disclose various embodiments of this technique. In the first patent just listed a first tackified image is formed on a transfer drum, the image is transferred to the first side of a copy sheet, and the sheet is inverted while the first tackified image dries and becomes fixed on the copy sheet. Thereafter, a second tackified copy image is formed on the transfer drum and the copy sheet is fed back into contact with the drum to transfer the second image to the second side of the copy sheet which is then transported to an output tray. In the latter three patents electrophotographic apparatus is disclosed for making copies wherein two images of an original are formed sequentially on a photoconductor, the images are developed and the first developed image is transferred to the first side of a copy sheet. The copy sheet is passed through a fuser to fuse or fix the first transferred image, is turned over and the opposite side of the copy sheet is brought into contact with the second developed image on the photoconductor. The second image is then transferred to the second side of the copy sheet, the copy sheet separated from the photoconductor and the second image fused by means of a second fuser. The disclosed techniques have several disadvantages. Since the first image is fixed before transfer of the second image, either two fusers must be used with attendant increase in cost, power and environmental heat or solvent fixing is used resulting in safety hazards and the inconvenience of handling consumable liquids.
In Research Disclosure Vol. 156, April, 1977, Item 15671 published by Industrial Opportunities Ltd. in Homewell, Havant, Hampshire, U.K., there is disclosed a single pass duplexing technique wherein unfixed toner images are transferred to opposite sides of a copy sheet before fixing of either image. A reversible vacuum belt inverts the copy sheet after transfer of the first image to the copy sheet.
A further disadvantage of many of the above duplexing techniques is the inability to handle heavy copy sheet stock. Most of these techniques are designed to handle copy sheets which are quite flexible and capable of being transported through relatively sharp paths. Heavier paper and card stock may be relatively inflexible and incapable of easily negotiating such sharp paths.